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Passive Thermal Management Options for EMS Devices

Monday, 01 April 2013

High-frequency pulsed electromagnetic stimulation (EMS)
devices are more powerful and effective than ever before.
These devices are finding applications in many areas, including
as treatments for stress and depression, osteoporosis, and soft tissue
injuries. Electromagnetic therapies stimulate tissue and cell
mass to recuperate faster. The base technology for pulsed electromagnetic
field (PEMF) is to input electrical energy into copper
windings to create a series of electromagnetic waves. The waves
offer a non-invasive anti-inflammatory and accelerated healing
treatment option. In many cases, these devices have a large metal
content and need to dissipate hundreds of watts of heat to effectively
generate and deliver pulsed electromagnetic waves.

Many of these PEMF devices require direct contact to the
patient’s skin to function. With repeated pulsing, the copper
windings generate waste heat. The more power that is input,
the more heat there is to be dissipated. The FDA mandates that
these devices cannot exceed 41°C at the patient contacting surface.
From the medical practitioner’s perspective, it would be
beneficial to be able to use these devices as frequently as needed,
without concern for exposing the
patient (or themselves) to a dangerously
hot device. The most cost-effective thermal
management solution is to use a heat
sink for natural convection. Device packaging
requirements and/or large metal
coils may exceed natural convection
capabilities and demand higher performance
thermal management solutions.
There are a number of both active and
passive cooling technologies available to
the medical device designer. More traditional
cooling solutions, such as pumped
liquid and forced convection air cooling,
work well in many applications. But,
these technologies require power to
operate, have a risk of leaking, and generate
noise. Fortunately, there are passive
thermal management solutions, such as
heat pipes and phase change materials,
that provide excellent thermal management performance
while requiring no input power and generating no noise.

As mentioned initially, EMS stimulation devices have a copper
coil structure consisting of a series of copper wire windings
wrapped around a solid metal core. The devices are pulsed,
generating electromagnetic waves when turned on. With repetitive
pulsing, the temperature of the device increases. This has
a direct impact on the patient contact surface temperature. In
many cases, the device must be turned off for several minutes
to allow heat to dissipate to a safe level prior to using again. An
effective thermal management system can resolve extended
downtime issues and passive technologies are a great option to
do that. An overview of heat pipes and phase change materials
is provided below.

■ Heat Pipes

Heat pipes are well established, reliable heat transfer
devices. Developed by Sandia National Laboratories in the
early 1960s, heat pipes are ubiquitous, found in laptop computers, satellites, high-powered electronics, as well as solar and
other alternative energy applications. They are sealed vacuum
devices that contain only a wicking material and a small
amount of a working fluid, typically water. A heat pipe is a two-phase
(liquid/vapor) device that efficiently transfers heat from
an external evaporator—the copper coils, in this case—to an
external condenser, a heat sink of some kind. Heat pipes transport
heat through boiling and condensation of the working
fluid. The heat from the evaporator causes the working fluid to
vaporize. Pressure pushes the vapor to the colder condenser
end, where it re-forms as a liquid and is absorbed by a wick
structure. The liquid is returned to the evaporator by capillary
action from the wick. A simple diagram showing how a heat
pipe operates is shown in Figure 1.

Heat pipes can be made in a variety of different sizes and
materials, and can accommodate many different power levels.
The most common system is a copper envelope/copper wick
with water as the working fluid. The typical maximum heat flux
for these copper water heat pipes is ~50-75 W/cm2, but can be
higher with specially designed wicks. The power capability for
a heat pipe is ~100W, and is dependent on heat pipe diameter,
length, wick structure, orientation vs. gravity, and other factors.
Heat pipes can be packaged together to handle larger heat
loads. Some of the advantages of heat pipes are that they can
be designed to work against gravity, and the freezing issue can
be solved with fluid inventory control. In addition, they can be
bent or flattened to accommodate different geometries.

In the case of EMS devices, heat pipes could be used to either
transport excess heat away from the metal core to a remotely
located heat sink, or to help spread the heat out over a larger
area, such as the device package envelope. Another option is to
combine heat pipes with phase change materials.

■ Phase Change Materials for Thermal Storage

Phase change materials (PCMs) are materials that can store
large amounts of heat when undergoing phase change from
solid to liquid. These materials have a high heat of fusion,
which is defined as the amount of heat (energy) required to
convert a solid at its melting point into a liquid, without an
increase in temperature. In PCM operation, when a pulsed,
powered device is turned “on”, heating causes the PCM to
begin melting. As the device continues to operate, the PCM,
now at its melting point, absorbs additional heat but does not
increase in temperature. When the device is turned “off”, the
PCM will dissipate heat and return to its original solid state.
PCMs are ideal for thermal management scenarios where transient
changes occur, such as in pulsing EMS devices. An important
design consideration is to ensure sufficient PCM is present
so that not all of it is melted during the “on” cycle.

The real benefit of PCMs is observed when examining multiple
cycles. Under repetitive cycling, the peak temperature can
be controlled to the melting point of the PCM, and dangerous
high peak temperatures can be avoided.

■ Choosing the Right PCM Material

To implement a PCM solution, it is critical to select the right
material. Desirable characteristics of a solid-liquid PCM
include high heat of fusion per volume and corresponding
melting and freezing characteristics to the specific application.
An interesting and effective thermal management solution for
electromagnetic stimulation devices is to combine heat pipes
and phase change materials. In this scenario, the heat from the
EMS device would be absorbed by the PCM. The stored heat
would then be transported by a heat pipe to a remotely located
heat sink. One of the many benefits of this approach is that
use of the PCM will also insulate the metal heat pipes from the
copper winding in the EMS, ensuring minimal distortion of
the electromagnetic wave generation.

Of course, specific package requirements will dictate if this
solution is feasible, and there are numerous other possible
combinations. The key point is that passive thermal management
solutions offer benefits that can greatly improve the duty
cycle of electromagnetic stimulation devices without requiring
any input power. All of these solutions can be used independently,
or with other technologies, and all offer the additional
benefits of long reliable life and silent operation.

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